Method for connecting magnetic substance target to backing plate, and magnetic substance target

ABSTRACT

A method for connecting a magnetic substance target to a backing plate with less variation in plate thickness, characterized in having the steps of connecting the magnetic substance target to an aluminum plate beforehand while maintaining the flatness, connecting the magnetic substance target connected to the aluminum plate to the backing plate while maintaining the flatness, and grinding out the aluminum plate, whereby the flatness of the magnetic substance target can be maintained until the magnetic substance target is connected to the backing plate by a relatively simple operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/498,146, which is the National Stage of International Application No.PCT/JP02/11863, filed Nov. 14, 2002, which claims the benefit under 35USC 119 of Japanese Application No. 2001-385666, filed Dec. 19, 2001.

BACKGROUND OF THE INVENTION

The present invention pertains to a method for connecting a magneticsubstance target with less variation in plate thickness to a backingplate, and to the magnetic substance target itself.

Conventionally, when performing magnetron sputtering to a ferromagneticsubstance target such as iron, cobalt, nickel, platinum or the alloysthereof upon disposing a magnet to the back of such a target, themagnetic field inside the target was blocked, and it was difficult togenerate a magnetic field on the target surface.

In other words, as a result of the magnetic field effect characteristicto ferromagnetic bodies such as iron, cobalt, nickel, platinum or thelike, the plasma density did not rise, the ionization efficiency ofargon deteriorated, and, as a result, the sputtering efficiency becamedeteriorated.

Thus, upon employing a ferromagnetic substance target such as iron,cobalt, nickel, platinum or the alloys thereof, measures were taken togenerate a magnetic field leak by simply making the target thicknessthin.

Generally, for the conventional connection of a magnetic substancetarget and a backing plate, a low-melting bonding material such asindium or the like was used.

Nevertheless, recently, this type of bonding material such as indiumwith weak bonding strength sometimes peels due to the influence of heatgenerated when latest high-power sputtering is employed. Thus, diffusionbonding has been proposed instead of this kind of bonding material, and,today, this diffusion bonding is becoming mainstream.

In light of the above, this is no exception to targets such as iron,cobalt, nickel, platinum and the alloys thereof, and diffusion bondingis similarly being performed thereto.

Meanwhile, although aluminum or aluminum alloy is generally used as thebacking plate, when using such aluminum or aluminum alloy, thedifference in the coefficient of thermal expansion between iron, cobalt,nickel, platinum or the alloys thereof and the aluminum or aluminumalloy will become significant, and, as a result, there were cases wherethe bonding interface would peel due to the increase in the camberingamount during the cooling process after diffusion.

In recent years, sputtering particles are being ionized with furtherhigh-power sputtering so as to perform even deposition while providinghigh kinetic energy to the substrate. As a result, the target to whichthe backing plate is connected deforms significantly in a convex shapedue to the thermal influence during sputtering and the water pressure ofthe cooling medium, and there were cases where water leakage wouldoccur.

Recently, the target itself is being enlarged pursuant to theenlargement of the bore diameter of wafers, and the peeling of thebonding interface and the deformation of the backing plate are becomingmajor problems. In consideration of the above, copper and copper alloyhaving stronger strength are being used as the backing plate.

Incidentally, with the ferromagnetic substance targets formed from iron,cobalt, nickel, platinum or the alloys thereof as described above, inorder to provide anisotropy to magnetism of the ferromagnetic substancetarget, complete annealing will not be performed thereto after finalplastic working, and it is necessary to leave in the as-wroughtstructure the processing stress caused by the plastic working.

Although it is one characteristic of a ferromagnetic substance target tohave such remnant stress, a major problem here is in that the remnantstress may cause cambering in the ferromagnetic substance targetmaterial such as iron, cobalt, nickel, platinum or the alloys thereofduring the bonding process to the backing plate. For instance, althoughheat of roughly 200 to 250° C. will be applied during the bondingprocess, cambering will occur even with the application of heat at suchlower temperatures.

In order to improve the sputtering efficiency of the magnetronsputtering target as described above, a target material having athickness of 10 mm or less, generally 5 mm or less is formed, butcambering appears notably with such a thin target, and there is aproblem in that this is extremely difficult to restore.

Generally, a ferromagnetic substance target formed from iron, cobalt,nickel, platinum or the alloys thereof is ultimately retained in avacuum chuck and subject to grinding. Even if the flatness thereof ismaintained during the retention in such chuck, if cambering occurs evenonce, a phenomenon of re-cambering may occur after the release of thechuck.

As described above, since a target material is thin, the problem ofcambering is serious, and there is a problem in that such targetmaterial cannot be easily connected to the backing plate whilemaintaining the flatness thereof.

SUMMARY OF THE INVENTION

In light of the above, an object of the present invention is to providea method for connecting a backing plate and a magnetic substance targetin which the flatness of the magnetic substance target can be maintaineduntil the magnetic substance target is connected to the backing plate bya relatively simple operation, as well as a magnetic substance targetwith less variation in plate thickness and leakage flux.

As a result of conducting intense study, the present inventors havediscovered that a disposable retention plate may be used to preventcambering.

Based on the foregoing discovery, the present invention provides amethod for connecting a magnetic substance target with less variation inplate thickness and a backing plate, comprising the steps of connectingthe magnetic substance target to an aluminum plate beforehand whilemaintaining the flatness, connecting the magnetic substance targetconnected to the aluminum plate to the backing plate while maintainingthe flatness, and grinding out the aluminum plate after the connectionof the magnetic substance target to the backing plate.

The magnetic substance target can be iron, cobalt, nickel, platinum orthe alloys thereof, and the backing plate can be copper, copper alloy,aluminum or aluminum alloy. Preferably, the magnetic substance targetand backing plate are connected via bonding or diffusion bonding, andthe diffusion bonding is performed via an insert material such as analuminum or aluminum alloy plate. Further, after grinding out thealuminum plate, the magnetic substance surface is preferably furtherground.

According to another aspect of the present invention, a magneticsubstance target is provided in which the thickness displacement inrelation to the average thickness is 4% or less.

According to yet another aspect of the present invention, a magneticsubstance target is provided in which, when the maximum leakage flux ofthe target is 100%, the average leakage flux in relation to the maximumleakage flux is 80% or more, or the minimum leakage flux in relation tothe maximum leakage flux is 70% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing the cross section of a casewhere a high purity cobalt target is placed on a vacuum chuck whilemaintaining a state of flatness, and an aluminum plate being connectedthereto via an adhesive material;

FIG. 2 is an explanatory diagram showing the cross section of a case ofconnecting a copper alloy backing plate to a high purity cobalt target,wherein an aluminum plate has been connected hereto via a bondingmaterial;

FIG. 3 is a diagram showing the relation between the integral powerconsumption and uniformity;

FIG. 4 is a diagram showing the results upon measuring the thickness ofthe nickel target in the case of employing an aluminum plate(reinforcing plate) at a total of 25 points for each prescribed angle(θ) radially from the center thereof with an ultrasonic thicknessindicator;

FIG. 5 is a diagram showing the measurement results of the leakage fluxof above;

FIG. 6 is a diagram showing the results upon measuring the thickness ofthe nickel target in the case of not employing an aluminum plate(reinforcing plate) at a total of 25 points for each prescribed angle(θ) radially from the center thereof with an ultrasonic thicknessindicator; and

FIG. 7 is a diagram showing the measurement results of the leakage fluxof above.

DETAILED DESCRIPTION OF THE INVENTION

In a method of forming an assembly comprising a magnetic substancetarget and a backing to the present invention, a magnetic substancetarget such as iron, cobalt, nickel, platinum or the alloys thereof(Fe—Ni alloy, Co—Ni alloy, Mn—Pt alloy, etc.) is first bonded to analuminum plate with the explosive bonding method, diffusion bondingmethod, brazing method, or other bonding methods while maintaining theflatness thereof by using a vacuum chuck or the like.

As the method of bonding to this aluminum plate, it will suffice so aslong as the bonding strength is sufficient at 250° C., and it isimportant that adverse effects are not inflicted upon the target. Assuch a method, explosive bonding method, diffusion bonding method,brazing method, or other bonding methods (adhesive methods) may be used.There is no particular limitation on these bonding methods, adhesives,and materials used for bonding.

The aluminum plate is in other words a reinforcing plate or protectiveplate for preventing cambering of the target plate bonded thereto. Thealuminum plate used here includes an aluminum alloy plate, and aninexpensive material may be used since it will be ground off later inthe process.

In order to maintain the flatness of the magnetic substance target, acertain degree of strength and thickness is required in the aluminumplate. Although it is appropriate to use an aluminum plate having athickness that is the same as or greater than the magnetic substancetarget, since the thickness of this aluminum plate may also bearbitrarily changed depending on the strength of amount of cambering ofthe magnetic substance target, there is no particular limitation on thethickness, and may be suitably selected.

Next, the magnetic substance target of the present invention as-bondedto the aluminum plate is bonded to the backing plate while maintainingthe flatness thereof. Here, this may be connected with conventionalbonding, or connected with diffusion bonding.

For example, when bonding with indium or indium alloy, a temperature of200 to 250° C. is applied, and it is necessary to make sure that theforegoing aluminum plate or bonding material has resistancecharacteristics against this temperature during the connection and thatcambering or the Like does not occur.

Since the temperature becomes relatively high during diffusion bonding,resistance characteristics against high temperatures are required. Forexample, in the case of a cobalt target, since low magnetic permeabilitymust be maintained, there is a limitation that the diffusion bondingmust be conducted at a low temperature (450° C. or less), but it isstill necessary to apply heat up to a temperature of several hundreddegrees.

Upon performing diffusion bonding, it is effective to use indium, or thealloy thereof, or other low-melting insert materials which have acertain degree of thickness. In some cases, aluminum or aluminum alloymay also be used. The function of the foregoing insert material is inthat diffusion bonding is enabled at low temperatures, and that theinsert material alleviates the stress generated by the difference ofthermal expansion between the target and backing plate after thediffusion bonding until it is cooled to room temperature.

It is preferable to use copper or copper alloy or aluminum or aluminumalloy which have stronger strength as the backing plate. For instance,in terms of a backing plate with less cambering during bonding and whichdoes not deform even during high power sputtering, it is effective touse copper alloys such as copper chrome alloy or copper zinc alloy asthe backing plate.

As described above, whether in the case of bonding or diffusion bonding,since the surface of the magnetic substance is covered with an aluminumplate (reinforcing plate or protective plate) via an adhesive material,the magnetic substance may be operated without being damaged by usingthe above as a protective plate during bonding until the completion ofthe connection of the magnetic substance to the backing plate, and, thismay also be pressed when necessary.

After the magnetic substance target is bonded to the backing plate, thealuminum plate as the reinforcing plate and the adhesive material areground off. Substances generated through the explosive bonding,diffusion bonding method, brazing, or other bonding or adhesive methodto the aluminum plate, and that is, the bonding or adhesive material andsubstances left or generated at the interface between the magneticsubstance target and the aluminum plate are all eliminated together withthe aluminum plate.

In this stage, since the magnetic substance target is connected to thebacking plate formed from the likes of aluminum alloy or copper alloyhaving stronger strength, the flatness thereof may be maintained withoutchange. After the grinding out of the aluminum plate and bondingmaterial, the magnetic substance surface may be further ground.

FIG. 1 is an explanatory diagram showing the cross section of a casewhere a high purity cobalt target 2 is placed on a vacuum chuck 1 whilemaintaining a state of flatness, and an aluminum plate 4 being connectedthereto via an bonding material 3.

FIG. 2 is an explanatory diagram showing the cross section of a case ofconnecting a copper alloy backing plate 5 to a high purity cobalt target2, wherein an aluminum plate 4 has been connected thereto via a bondingmaterial 3, with a bonding brazing material 6.

As a result of employing the foregoing method for connecting a magneticsubstance target to a backing plate, obtained is a magnetic substancetarget in which a variation in the thickness of the magnetic substancetarget is 4% or less of an average thickness thereof; the averageleakage flux in relation to the maximum leakage flux is 80% or more,preferably 90% or more, when the maximum leakage flux of the target is100%; and the minimum leakage flux in relation to said maximum leakageflux is 70% or more when the maximum leakage flux of the target is 100%at the respective position across the surface area of the target.

This magnetic substance target may be employed as a magnetic substanceof iron, cobalt, nickel, platinum or the alloys thereof.

Further, the leakage flux may be measured upon using a standard gaussmeter. In other words, a magnet is placed on the backing plate side, aprobe is made to contact the magnetic substance side on the oppositeside, and measurement is thereby made with a gauss meter. The positionto be measured is performed by arbitrarily moving the probe.

EXAMPLES AND COMPARATIVE EXAMPLES

The present invention is now explained in detail with reference to theExamples. These Examples are merely illustrative, and the presentinvention shall in no way be limited thereby. In other words, variousmodifications and other embodiments based on the technical spiritclaimed in the claims shall be included in the present invention as amatter of course.

Example 1

High purity cobalt raw material of 99.999 wt % (5N) was warm rolled at450° C. to prepare a high purity cobalt plate having a thickness of 6mm, and this was further machine processed to complete a discoid targethaving a diameter of φ350 mm and a thickness of 3.5 mm.

Copper chrome alloy (chrome content of 1 wt %) was used as the backingplate.

While retaining the high purity cobalt plate with a vacuum chuck andmaintaining the flatness, an aluminum plate of 10 mm was connected tothis high purity cobalt plate with PbAgSn brazing material(97.5Pb-1Sn-1.5Ag) at 325° C. After the connection, the target side wassurface ground (approximately 0.3 mm) so as to form a flat surface.

Next, indium soldering was used to connect the sputtering target and thebacking plate. The heating temperature was 230° C.

Thereafter, the aluminum plate was removed with mechanical processing(grinding), and the cobalt was faced (approximately 0.2 mm) so as toobtain a target-backing plate assembly.

Next, with the discoid cobalt target facing upward, the target thicknesswas measured with a supersonic thickness indicator. A total of 49 points(1 point in the center, 8 points in ⅓ of the periphery, 16 points in ⅔of the periphery, and 24 points in the outer periphery) were radiallymeasured.

As a result, the maximum thickness was 3.06 mm, the minimum thicknesswas 2.90 mm, and the displacement from the target thickness was 0.1 mm(3.3%) at maximum. Further, the difference between the maximum thicknessand minimum thickness was 0.16 mm. As described above, the thicknessvariation was small, the connection status was favorable, and there wereno generation of abrasion marks or the like.

Although cambering should occur during the connection of the target tothe backing plate or during the processing thereof, this is firmlyretained and protected with the aluminum plate via the brazing material.Further, grinding of aluminum and brazing material is easy, and theincrease in processing steps or costs was minimal.

Next, the discoid cobalt target-backing plate assembly was used toperform sputtering on a substrate, and the uniformity of the formedcobalt was observed. The results are shown in FIG. 3.

Moreover, the sputtering conditions were as follows:

Applied Power: 1 kw

T-S: 50 mm

Film Thickness: 1000 Å

Ar Pressure: 9×10⁻³ Torr

As shown in FIG. 3, the uniformity begins to improve from around anintegral power consumption of approximately 6 kwh, this is maintained ata uniformity of 2% or less up to an integral power consumption ofapproximately 30 kwh, and this shows that the uniformity of the filmformed with sputtering is favorable. This is considered to be a resultof the variation in the target thickness being small, and the flatnessbeing superior.

Comparative Example 1

With the same method as Example 1, high purity cobalt raw material of99.999 wt % (5N) was warm rolled at 450° C. to prepare a high puritycobalt plate having a thickness of 6 mm, and this was further machineprocessed to complete a discoid target having a diameter of φ350 mm anda thickness of 3.0 mm. Copper chrome alloy (chrome content of 1 wt %)was used as the backing plate, and indium soldering was used to directlyconnect the sputtering target and backing plate. The heating temperaturewas 230° C.

Next, similar to Example 1, with the discoid cobalt target facingupward, the warp amount was measured with a supersonic thicknessindicator. A total of 49 points (1 point in the center, 8 points in ⅓ ofthe periphery, 16 points in ⅔ of the periphery, and 24 points in theouter periphery) were radially measured.

As a result, the maximum thickness was 3.12 mm, the minimum thicknesswas 2.78 mm, and the displacement from the target thickness was 0.22 mm(7.3%) at maximum. Further, the difference between the maximum thicknessand minimum thickness was 0.34 mm.

As described above, the warp amount was significantly large, and therewere generation of abrasion marks and the like. This is considered to bebecause since a reinforcing aluminum plate is not provided, significantcambering occurred during the connection of the sputtering target andbacking plate, and, since grinding was conducted with the existence ofsuch camber, the center portion became thin and the outer portion becamethick (the opposite for the grinding side).

Next, the discoid cobalt target-backing plate assembly was used toperform sputtering on a substrate, and the uniformity of the formedcobalt was observed. The results are similarly shown in FIG. 3 incomparison to Example 1.

Incidentally, the sputtering conditions were the same as Example 1.

As shown in FIG. 3, uniformity was inferior at roughly 7% even at anintegral power consumption of approximately 6 kwh. And, the uniformityexceeded 2% up to an integral power consumption of approximately 30 kwh,and varied.

The reason why the uniformity of the film formed upon sputtering thediscoid cobalt target illustrated in the Comparative Example isconsidered to be because the variation in the target thickness beinggreat, and the flatness being inferior.

Example 2

High purity nickel raw material of 99.999 wt % (5N) was warm rolled at450° C. to prepare a high purity nickel plate having a thickness of 6mm, and this was further machine processed to complete a discoid targethaving a diameter of φ350 mm and a thickness of 3.5 mm. Copper chromealloy (chrome content of 1 wt %) was used as the backing plate.

While retaining the high purity nickel plate with a vacuum chuck andmaintaining the flatness, an aluminum plate of 10 mm was connected tothis high purity nickel plate with PbAgSn brazing material(97.5Pb-1Sn-1.5Ag) at 325° C. After the connection, the target side wassurface ground (approximately 0.3 mm) so as to form a flat surface.

Next, indium soldering was used to connect the sputtering target and thebacking plate. The heating temperature was 230° C.

Thereafter, the aluminum plate was removed with mechanical processing(grinding), and the nickel was faced (approximately 0.2 mm) so as toobtain a target-backing plate assembly.

Next, with the discoid nickel target facing upward, the target thicknesswas measured with a supersonic thickness indicator. A total of 25 points(1 point in the center, 8 points in ⅓ of the periphery, and 16 points in⅔ of the periphery) were measured for each prescribed angle (θ) radiallyfrom the center thereof. The results are shown in Table 1. Further, thegraph corresponding to Table 1 is shown in FIG. 4.

As a result, as shown in Table 1 and FIG. 4, the maximum thickness was3.24 mm, the minimum thickness was 3.20 mm, and the average was 3.216mm. The displacement from the average thickness was 0.024 mm (0.7%) atmaximum. Further, the difference between the maximum thickness andminimum thickness was 0.04 mm. As described above, the thicknessvariation was extremely small, and, as with Example 1, the connectionstatus was favorable, and there were no generation of abrasion marks orthe like.

TABLE 1 θ(deg) Center ⅓R ⅔R 0 3.21 0 3.20 45 3.20 90 3.20 135 3.20 1803.22 225 3.21 270 3.21 315 3.21 0 3.22 22.5 3.21 45 3.21 67.5 3.21 903.20 112.5 3.21 135 3.23 157.5 3.22 180 3.23 202.5 3.24 225 3.22 247.53.22 270 3.23 292.5 3.23 315 3.22 337.5 3.23

Next, the leakage flux of this discoid nickel target was measured with agauss meter. The points measured were the same as the thicknessmeasurement described above. Generally, the larger the leakage flux, thehigher the sputtering efficiency, and this is considered to befavorable.

The relative values in the respective measuring points when thestrongest value of the leakage flux is 100% were measured. The resultwas maximum 100%, minimum 91%, and average 95%. The difference betweenmaximum and minimum was 9%. The results are shown in Table 2 and FIG. 5.

In comparison to Comparative Example 2 described later, a result wasobtained in that the leakage flux will increase if the change inthickness is small. Further, in the case of nickel, it has becomeevident that the leakage flux will be influenced significantly due tothe variation in thickness.

TABLE 2 θ(deg) Center ⅓R ⅔R 0 94% 0 97% 45 98% 90 99% 135 100% 180 96%225 93% 270 95% 315 94% 0 92% 22.5 96% 45 97% 67.5 96% 90 100% 112.5100% 135 93% 157.5 92% 180 93% 202.5 91% 225 94% 247.5 95% 270 93% 292.592% 315 93% 337.5 91%

The strongest value of the leakage flux is indicated as 100%.

Comparative Example 2

With the same method as Example 2, high purity nickel raw material of99.999 wt % (5N) was warm rolled at 450° C. to prepare a high puritynickel plate having a thickness of 6 mm, and this was further machineprocessed to complete a discoid target having a diameter of φ350 mm anda thickness of 3 mm. Copper chrome alloy (chrome content of 1 wt %) wasused as the backing plate, and indium soldering was used to directlyconnect the sputtering target and backing plate. The heating temperaturewas 230° C.

Next, similar to Example 2, a total of 25 points (1 point in the center,8 points in ⅓ of the periphery, and 16 points in ⅔ of the periphery)were measured for each prescribed angle (θ) radially from the centerthereof. The results are shown in Table 3. Further, the graphcorresponding to Table 3 is shown in FIG. 6.

TABLE 3 θ(deg) Center ⅓R ⅔R 0 2.95 0 3.07 45 3.10 90 3.05 135 3.11 1803.12 225 3.08 270 3.10 315 3.12 0 3.25 22.5 3.22 45 3.21 67.5 3.21 903.20 112.5 3.21 135 3.31 157.5 3.29 180 3.30 202.5 3.29 225 3.27 247.53.25 270 3.23 292.5 3.23 315 3.22 337.5 3.19

As a result, as shown in Table 3 and FIG. 6, the maximum thickness was3.31 mm, the minimum thickness was 2.95 mm, and the average was 3.183mm. The displacement from the average thickness was 0.233 mm (7.3%) atmaximum. Further, the difference between the maximum thickness andminimum thickness was 0.36 mm.

As described above, the warp amount was significantly large, and therewere generation of abrasion marks and the like as with ComparativeExample 1. This is considered to be because, since a reinforcingaluminum plate is not provided, significant cambering occurred duringthe connection of the sputtering target and backing plate, and, sincegrinding was conducted with the existence of such camber, the centerportion became thin and the outer portion became thick (the opposite forthe grinding side).

Next, the leakage flux of this discoid nickel target was measured with agauss meter. The points measured were the same as the thicknessmeasurement described above.

The relative values in the respective measuring points when thestrongest value of the leakage flux is 100% were measured. The resultwas maximum 100% (center portion), minimum 64%, and average 79.5%. Thedifference between maximum and minimum was 36%. The results are shown inTable 4 and FIG. 7.

In comparison to Comparative Example 2 described above, a result wasobtained in that the larger the change in thickness, the faster theleakage flux decreases.

TABLE 4 θ(deg) Center ⅓R ⅔R 0 100% 0 91% 45 89% 90 92% 135 89% 180 86%225 88% 270 90% 315 88% 0 69% 22.5 75% 45 73% 67.5 72% 90 76% 112.5 77%135 64% 157.5 67% 180 66% 202.5 70% 225 73% 247.5 75% 270 77% 292.5 78%315 80% 337.5 83%

The present invention yields a superior effect in that, upon connectinga magnetic substance target to a backing plate, the flatness of themagnetic substance target during operation can be maintained until themagnetic substance target is connected to the backing plate by arelatively simple operation, and without generating any cambering of themagnetic substance target, and the flatness can be maintained afterconnecting a magnetic substance target and a backing plate.

The invention claimed is:
 1. An assembly, comprising: a magneticsubstance target, a backing plate, and an aluminum reinforcing plate,said magnetic substance target having an as-wrought structure withremnant stress sufficient to provide magnetic anisotropy necessary forenabling magnetron sputtering, wherein said magnetic substance targethas a surface face, an opposite face bonded to said backing plate, and athickness defined by a distance between said surface face and saidopposite face, the thickness being 5 mm or less on average along saidsurface face and having a variation of 4% or less of said averagethickness, wherein the surface face is bonded to the aluminumreinforcing plate such that the aluminum reinforcing plate is integralto the magnetic substance target, and wherein the opposite face isbonded to the backing plate after bonding the surface face to thereinforcing plate.
 2. An assembly according to claim 1, wherein themagnetic substance target is made of a magnetic substance selected fromthe group consisting of iron, cobalt, nickel, platinum and alloysthereof.
 3. The assembly according to claim 2, wherein the surface faceis bonded to the aluminum reinforcing plate with an adhesive or brazingmaterial.